device type + one small fix

hey,

I found your package for django mobile and I made a few modifications, i hope those are useful and let me know about any feedback, I would really appriciate it 

* ) I added methods to support the device type of the phone, so the template and views can know which device the page was accessed from

* ) unless I am mistaken, the middleware checks the user agent for every load, I modified it to be once the session is started, I don't think a check for every page load is mandatory.

Signed-off-by: Max Presman <max@presman.ca>